<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html{
            margin: 20px;
        }
        .fixed{
            position: fixed;
            left: 0px;
            top: 0px;
            bottom: 0px;
            right: 0px;
            background-color: lightgreen;
        }
    </style>
</head>
<body>
    <div class="fixed"></div>
</body>
</html>